This new post on Cormac's blog shows a little trick you can use to make "read-only" object variables with the help of the handy magic methods built in to PHP5.

You can create read-only object variables by using the "private" keyword and the __get() and __set() magic methods. [...] So now classWithReadOnlyVar::readOnlyVar is only settable from inside the class, but you can read it from anywhere.

His example code initially sets up the read-only variable as a property of the example class. The __get magic method is called to correctly fetch the value but the __set intercepts anything trying to change its value. This same sort of thing can be accomplished with the protected/private keywords in PHP5.